Heat insulating articles which are prepared via a dry process and which consist of silicon dioxide having a surface area of at least 50 m.sup.2 /g, opacifiers and optionally, inorganic fibers are described in German Pat. No. 2,036,124.
Heretofore, heat insulating articles prepared from silicon dioxide had a tendency to deteriorate when exposed to water. However, if heat insulating articles are prepared from inorganic fibers, opacifiers and silicon dioxide having a surface area of at least 50 m.sup.2 /g which has been treated with for example trimethylethoxysilane to render the same partially or wholly hydrophobic, the resultant heat insulating articles have poor mechanical strength. Moreover, if one were to prepare waterrepellent heat insulating articles via a dry process using inorganic fibers, opacifiers and silicon dioxide having a surface area of at least 50 m.sup.2 /g which has been treated with, for example trimethylethoxysilane to render the same partially hydrophobic, the resultant heat insulating articles are somewhat water repellent, but lack mechanical strength. Likewise when a mixture containing untreated silicon dioxide and more or less completely hydrophobic silicon dioxide having a surface area of at least 50 m.sup.2 /g is used in the preparation of heat insulating articles, these articles have very little mechanical strength. Thus, it would be desirable to have heat insulating articles with good mechanical strength which will not deteriorate when exposed to water.
Therefore, it is an object of this invention to provide heat insulating articles which are hydrophobic and have good mechanical strength. Another object of this invention is to provide heat insulating articles which will not deteriorate when exposed to water. Still another object of this invention is to provide a process for treating heat insulating articles to improve their properties. A further object of this invention is to provide a process for treating heat insulating articles to render them hydrophobic.